decoder circuit

Posted on Feb 4, 2014

As you can see from the article LPT port expander, this PPI-8255 is a general purpose I/O programmable function. But usually this IC are tired to the computer bus directly. Return to this idea, here I will show you that the card can be used to dual function purpose. Beside for the expander port, it can be used for I/O functional directly. So we c

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an used the card both for the expander experiment or for direct connection to the computer bus experiment. Why do you need this I/O directly connection for Many article on the net usually used this method for I/O interfacing. So by doing this, we can take 2 benefits. First, we can used many useful routines (usually with the tested ones) or directly used the softwares ( ) or routines; secondly, we can used many hardware design by someone else (compatibility type). At most we can replace the I/O parts. Happy programming and happy surfing on the net. To directly connected a hardware to computer bus, we need an I/O decoding. Here I don`t explain anymore, please refer to the topic of decoding technique at the other page. There are 2 type of decoding technique, ie: partial decoding, when we want to reduce the components using or full decoding. I prefer the last one. The decoder circuit must be preserve all of the possible unused I/O ports, easy to select the address, and not ambiguous for the future compatibility design. I used a comparator 74LS682 type and a dip switch for this purpose. Besides that the lines need buffers because the card may put far away from the PC. The complete circuit diagram shown at fig-1. One thing must remember that, when doing re-boot-ing the PC, the expander card must be connected to this decoder card. So the PPI can be reset. If not, the PPI can not be initialized. To anticipate this purpose, we can...

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